Variable evaporator control for a gas dryer

ABSTRACT

A variable evaporator control system and method in a gas dryer for maximizing the cooling which can be accomplished for a given length heat exchanger by adjusting the evaporator refrigerant approach temperature responsive to changes in the gas load on the system. Pressure and/or temperature sensors positioned at particular locations in the system provide feedback for controlling adjustments in the approach temperature depending on the gas load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 10/123,565,filed on Apr. 16, 2002, now U.S. Pat. No. 6,711,906 which claimspriority to U.S. provisional patent application Ser. No. 60/285,528,filed Apr. 20, 2001, the disclosure of which is hereby incorporated byreference.

BACKGROUND

The invention relates generally to gas dryers, and more particularly toa variable evaporator control (“VEC”) system and method for arefrigerated compressed gas dryer which provides for varying therefrigerant temperature responsive to changes in the compressed gas loadon the refrigerant compressor.

Refrigerated compressed gas dryers are used to remove moisture and watervapor from compressed gas streams which are introduced through the gascompressor intake when the compressed gas is taken from the ambient withits accompanying relative humidity. Once the gas is compressed, itsvapor holding capacity is reduced and the vapor condenses into a liquidas the gas is cooled. Prior art type refrigerated compressed gas dryersbasically consist of a pre-cooler/re-heater heat exchanger, anevaporator heat exchanger, a liquid separator and a liquid drain valve.The warm compressed gas is passed through the pre-cooler/re-heater whereit is cooled by the outgoing cooled gas stream. As the warm compressedgas stream is cooled moisture vapor begins to condense into a liquid.The compressed gas stream is then passed into the evaporator heatexchanger where it is further cooled to a lower temperature as therefrigerant evaporates at some temperature below the desired temperatureof the compressed gas stream exiting the evaporator. More water vapor iscondensed into a liquid state in the evaporator and the cooled gasstream is passed to the liquid separator where the condensed liquid isseparated and removed from the system by the drain valve. The cooled anddried compressed gas stream is then returned through thepre-cooler/re-heater to pre-cool the warm incoming compressed gas streamprior to being returned to the compressed gas system piping. Compressedgas flow rates will vary as a function of time in nearly everycompressed gas dryer application. The equipment can be expected toexperience flows ranging from the maximum design flow rate down to ano-load, or zero, flow rate condition.

The refrigeration system of a typical refrigerated compressed gas dryeras described above basically consists of a refrigerant compressor, arefrigerant condenser, an expansion/restrictive device, and theevaporator described above. The temperature of the cooled compressedgas, as it exits the evaporator, defines the thermal performance ofcompressed gas dryers. This is typically expressed at the design flowrate. Increased cooling of the warm compressed gas results in lowerexiting evaporator compressed gas temperatures and higher levels ofmoisture removal. However, there is a practical limit to the amount ofcooling that can be done in the evaporator of a refrigerated gas dryer.Cooling the warm compressed gas stream down to a temperature below thefreezing point of water creates a situation where the condensate canfreeze and block the free path of the compressed gas stream, thus,increasing the pressure drop across the dryer. In extreme circumstances,the flow can be blocked completely, starving the downstream process ofcompressed gas. This failure situation will most likely occur duringcompressed gas flow rates that are much less than the maximum designflow rate. When using evaporators constructed from smooth tubing, thefreeze-up failure potential necessitates that the refrigeranttemperature in the evaporator be above the freezing point of water, andheld fixed and steady, as the load varies from no load to full load. Allmanufacturers of refrigerated compressed gas drying equipment mustaddress how to control the evaporator refrigerant temperature in orderto prevent condensate freeze-up under low or no load operatingconditions, while providing the thermal performance advertised at a fullload situation.

Presently, the most common method of controlling the evaporatorrefrigerant temperature in the compressed gas dryer is through the useof a hot gas by-pass valve, which is a pressure-regulating valve that isset to maintain a constant refrigerant pressure in the evaporator andrefrigerant compressor suction line. The by-pass valve operates bymetering high-pressure refrigerant discharge gas into the refrigerantcompressor suction line whenever the suction pressure drops below theset point of the pressure regulating by-pass valve. By understanding thesaturation temperature/pressure correlation of the refrigerant gas, theevaporator refrigerant temperature can be indirectly regulated bymaintaining a constant refrigerant suction pressure. Thistemperature/pressure correlation refers to the unique physicalsaturation properties of each refrigerant; that is, as a refrigerantchanges phase from a liquid to a vapor (i.e., boils or evaporates), itwill do so at a constant temperature and pressure. If the pressure iscontrolled and maintained while this phase change occurs, thetemperature is also maintained. Therefore, the more precisely thepressure is maintained, the more accurately the evaporator temperature:is held constant. A typical pressure setting for the by-pass valve wouldbe a refrigerant saturation pressure that corresponds to a saturationtemperature of approximately 35 degrees Fahrenheit. Placing theequivalent temperature setting slightly above the freezing point ofwater allows for a small factor of safety in the event of any valvesetting drift.

Another commonly used method to maintain a constant refrigerant suctionpressure is to install an automatic pressure valve (“APV”) in place ofthe expansion/restrictive device and the hot gas by-pass valve. The APVmaintains proper refrigerant suction pressure by metering high-pressureliquid refrigerant into the inlet of the evaporator. The APV istypically inexpensive and inaccurate. Under no-load conditions, theliquid refrigerant may not be effectively converted into a gas in theevaporator, which can result in a liquid flood-back condition at therefrigerant compressor suction, with potential compressor damage. Also,as the load is applied to the dryer, the refrigerant suction pressureoften increases, resulting in poor thermal performance. Some of thenewer technologies used to maintain a constant refrigerant suctionpressure include the use of variable speed refrigerant compressors whichoperate by altering the rotational speed, and therefore, the pumpingcapacity of the compressor. The refrigerant suction pressure can beincreased or decreased by decreasing or increasing, respectively, therotational speed of the compressor. Regardless of the manner ofcontrolling the suction pressure, typical prior art control schemesfunction to maintain a constant suction pressure, and thus a constantevaporator refrigerant temperature, regardless of the load on thecompressor. Consequently, prior art methods can suffer problems such aslower efficiency or freeze up conditions during compressor no-loadconditions.

Many conventional compressed gas dryers utilize smooth tubes in theevaporator, which offer the advantage of a non-fouling surface thatperforms consistently throughout the life of the dryer. Other advantagesare reduced pressure drop and relatively inexpensive manufacturingcosts. A disadvantage of smooth tube technology is that a relativelylarge amount of heat exchange surface is necessary in order to achievethe desired thermal performance at the design full load condition. Thiscan be particularly challenging when considering the no-load and partialload freeze up concerns discussed previously, as well as the need tooperate the evaporator at 35 degrees Fahrenheit, offering a 4 degreeFahrenheit approach temperature. The efficient packaging of these dryerscan be inherently more difficult. Extended surface heat exchanger tubesare often used in order to make the evaporator more compact. Theexternally finned surface of such designs offer a temperature gradientbetween the refrigerant and the compressed gas stream. This gradient canpermit the refrigerant temperature to be less than the freezing point ofwater, without the danger of freeze-up. A reduced refrigeranttemperature results in a larger temperature approach, and less requiredsurface area. While the length required for this design is reduced ascompared to the smooth tube designs, the cost of the tube, and thedesign, can generally be greater. The designer may also have to addressthe concerns of excessive pressure drop.

Small, compact heat exchangers, such as brazed plate, or bar and frametype heat exchangers, offer an extremely attractive packaging solutionfor a compressed gas dryer, but, again, can be much more costly than thesmooth tube designs. As these designs do not incorporate extendedsurfaces and the above discussed temperature gradients, the refrigeranttemperatures must remain above the freezing point of water in order toperform reliably under all operating conditions. A precise and constantevaporator refrigerant temperature control is imperative to thesedesigns.

Due to the factors explained above, there has generally been no singleoptimum heat exchanger design for a compressed gas dryer. A problem hasbeen that prior art designs are configured to maintain a constantsuction pressure, and thus evaporator refrigerant temperature,regardless of the compressed gas load on the refrigerant compressor.Consequently, there has been a compromise between the desired featuresof thermal performance, pressure drop performance, reliable operation,size, cost and packaging. The shortcomings of prior art refrigeratedcompressed gas systems described above illustrates the need for acontrol system for a refrigerated compressed gas dryer which can varythe evaporator refrigerant temperature in response to changes in theload on the refrigerant compressor. Consequently, the coolingcapability, per-unit length, of any given length heat exchanger can bemaximized.

SUMMARY

A variable evaporator control system and method are provided foradjusting the evaporator refrigerant temperature responsive to changesin the load on the refrigerant compressor in a refrigerated compressedgas dryer. A control system according to the invention can utilize, forexample, pressure and temperature sensors, a pair of temperaturesensors, or a single appropriately positioned temperature sensor. Eachof the sensors can be positioned at preselected locations in the systemto provide feedback to a processor which can analyze the output in orderto determine whether to increase or decrease the approach temperature,i.e., the difference between the temperature of the warm gas and therefrigerant temperature at the inlet of the heat exchanger. The controlsystem can preferably include at least one temperature sensor formonitoring the temperature of the refrigerant at the evaporator. Therefrigerant suction pressure can be controlled to vary the temperatureof the refrigerant at the evaporator inlet to generally maintain adesired outlet compressed gas temperature irrespective of the load onthe refrigerant compressor. In this way, the temperature of the driedcompressed gas exiting the evaporator is generally maintained whilemaking efficient use of the evaporator. For example, the evaporator canhave a shorter effective length and still provide the desired level ofcooling both at maximum design load for the evaporator and also duringlow or zero load on the refrigerant compressor. This can be accomplishedwhile avoiding potential freeze up problems which conventionally occurin systems which maintain a generally constant suction line pressureregardless of the load on the compressor. Moreover, this can beaccomplished using a smooth tube evaporator with all of the attendantadvantages while avoiding the potential freeze up problems which can beproblematic with smooth tube designs.

According to the invention, the refrigerant suction pressure can beadjustably controlled in different ways, including, for example, usingan electrically adjustable by-pass valve, varying the speed of avariable speed compressor, or using an unloading compressor arrangement.Adjustments in the refrigerant temperature at the inlet of the heatexchanger can be made generally in response to changes in the load onthe compressor. In particular, a lower refrigerant temperature can bemaintained where there is a high load on the compressor. However, as theload on the compressor decreases, the refrigerant temperature can beadjusted upwards, in order to avoid potential freeze up problems whichcould occur if the compressed gas temperature were reduced below thefreezing point of water. In a presently preferred embodiment,temperature can be sensed at a single point in the system wherein thetemperature is indicative of the load on the compressor. Feedback fromthis single point temperature sensor can be utilized to adjust theapproach temperature depending on the load on the compressor.

Other details, objects, and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawing FIGS. of certain embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein:

FIG. 1 illustrates diagrammatically a prior art refrigerated compressedgas dryer which uses a by-pass valve to regulate suction pressure.

FIG. 2 illustrates diagrammatically a prior art refrigerated compressedgas dryer which uses an automatic pressure valve to regulate suctionpressure.

FIG. 3.1 graphically illustrates temperature profiles of prior artcompressed gas dryers, such as shown in FIGS. 1 and 2, employing a 4degree approach temperature.

FIG. 3.2 graphically illustrates temperature profiles of prior artcompressed gas dryers, similar to FIG. 3.1, except employing a 14 degreeapproach temperature.

FIG. 3.3 graphically illustrates temperature profiles of a compressedgas dryer utilizing a control system and method according to theinvention.

FIG. 4 illustrates diagrammatically a presently preferred embodiment ofthe invention using pressure and temperature sensors.

FIG. 5 illustrates diagrammatically an alternative embodiment of theinvention using two temperature sensors.

FIG. 6 illustrates diagrammatically an alternative embodiment of theinvention using a single temperature sensor.

FIG. 7 illustrates an embodiment of a smooth tube evaporator design anda presently preferred embodiment of an apparatus for a single pointtemperature sensing control system and method for use with theembodiment of the invention shown in FIG. 6.

FIG. 8 illustrates diagrammatically an alternative embodiment of theinvention similar to that shown in FIG. 4 except using an unloadingcompressor.

FIG. 9 illustrates diagrammatically an alternative embodiment of theinvention similar to that shown in FIG. 5 except using an unloadingcompressor.

FIG. 10 illustrates diagrammatically an alternative embodiment of theinvention similar to that shown in FIG. 6 except using an unloadingcompressor.

FIG. 11 illustrates diagrammatically an alternative embodiment of theinvention similar to that shown in FIGS. 4 and 8, except using avariable speed compressor.

FIG. 12 illustrates diagrammatically an alternative embodiment of theinvention similar to that shown in FIGS. 5 and 9, except using avariable speed compressor.

FIG. 13 illustrates diagrammatically an alternative embodiment of theinvention similar to that shown in FIGS. 6 and 10, except using avariable speed compressor.

DETAILED DESCRIPTION

Before describing the invention, a more detailed description of priorart type refrigerated compressed gas dryers is provided for ease ofunderstanding more clearly the advantages of the invention. Referring toFIG. 1, a prior art type refrigerated compressed gas dryer 15 is shownbasically consisting of, as described in the background, apre-cooler/re-heater heat exchanger 21, a gas-to-refrigerant evaporatorheat exchanger 22, a liquid separator 23 and a single or multiple liquiddrain valve(s) 24. The incoming warm compressed gas 17, which containswater vapor, flows into the pre-cooler/re-heater 21, where it is cooledby the outgoing cold gas stream 19. The pre-cooler/re-heater 21 helps toreduce the heat load placed on the refrigeration system. As the warmcompressed gas stream 17 is cooled, the moisture vapor begins tocondense into a liquid. The compressed gas and condensed moisture thenleaves the pre-cooler section of the pre-cooler/re-heater 21 and entersthe compressed gas inlet 46 of the gas-to-refrigerant evaporator heatexchanger 22. Here the warm compressed gas stream 17 is additionallycooled to a lower temperature, usually 39 degrees Fahrenheit, as therefrigerant evaporates at some temperature (usually about 35 degreesFahrenheit) below the desired temperature of the compressed gas streamexiting the compressed gas outlet 48 of the evaporator 22. Again, morewater vapor is condensed into a liquid state. Exiting the evaporator 22,the cooled compressed gas stream 19 flows into the liquid separator 23,where the condensed liquid is separated from the cooled compressed gasstream 19. After separation, this liquid is collected and removed fromthe system entirely by one, or many, liquid drain valves 24. The cooled,and liquid-free compressed gas stream 19 then exits the separator andre-enters the pre-cooler/re-heater 21. Here the cooled compressed gasstream 19 is re-heated by transferring heat with the warm incomingcompressed gas stream 17. The reheated compressed gas stream 19 thenexits the dryer 15 and continues flowing through the compressed gassystem piping (not shown). Compressed gas flow rates will vary as afunction of time in nearly every compressed gas dryer application. Theequipment can be expected to experience flows ranging from the maximumdesign flow rate down to a no load, or zero flow rate, condition.

The refrigeration system 16 in a typical compressed gas dryer 15 is alsoshown in FIG. 1. In its basic form, it consists of a refrigerationcompressor 25, a refrigerant condenser 26, an expansion/restrictivedevice 27, and the gas-to-refrigerant evaporator heat exchanger 22described above. The restrictive devices 27 may include capillary tubes,thermal expansion valves (TXV), orifices, electronic expansion valves,and other devices known in the art. The temperature of the cooledcompressed gas 19, as it exits the evaporator 22, defines the thermalperformance required of compressed gas dryers. This is typicallyexpressed at the design flow rate. Increased cooling of the warmcompressed gas 17 results in lower exiting evaporator 22 compressed gastemperatures and higher levels of moisture removal. However, there is apractical limit to the amount of cooling that can be done in theevaporator 22 of a refrigerated gas dryer. Cooling the warm compressedgas stream 17 down to a temperature below the freezing point of watercreates a situation where the condensate can freeze and block the freepath of the compressed gas stream, thus, increasing the pressure dropacross the dryer 15. In extreme circumstances, the flow can be blockedcompletely, starving the downstream process of compressed gas. Thisfailure situation will most likely occur during compressed gas flowrates that are much less than the maximum design flow rate. When usingevaporators constructed from smooth tubing, the freeze-up failurepotential necessitates that the refrigerant temperature in theevaporator 22 be above the freezing point of water, and held fixed andsteady, as the load varies from no load to full load. All manufacturersof refrigerated compressed gas drying equipment must address how tocontrol the evaporator 22 refrigerant temperature in order to preventcondensate freeze-up under low or no load operating conditions, whileproviding the thermal performance advertised at a full load situation. Adescription of some of these methods follows.

As explained above in the background section, some prior art methods ofcontrolling the evaporator 22 refrigerant temperature do so indirectlyby maintaining a generally constant pressure in the evaporator 22 andsuction line 31. The pressure being maintained in the suction line 31generally corresponds to a predetermined refrigerant pressure to beprovided at the evaporator. The most common manner of regulating thesuction pressure is using a hot gas by-pass valve 28. The by-pass valve28 is a pressure-regulating valve that can be set to maintain a constantrefrigerant suction pressure. The by-pass valve 28 meters high-pressurerefrigerant discharge gas into the suction line 31 whenever the suctionpressure drops below the set point of the by-pass valve 28. Byunderstanding the saturation temperature/pressure correlation of therefrigerant gas, the evaporator 22 refrigerant temperature is indirectlycontrolled by maintaining a constant refrigerant suction pressure. Themore precisely the suction pressure is maintained constant, the moreaccurately a constant evaporator 22 temperature is held. A typicalpressure setting for the by-pass valve 28 would be a refrigerantsaturation pressure that corresponds to 35 degrees Fahrenheit. Placingthe equivalent temperature setting above the freezing point of waterallows for a small factor of safety in the event of any valve settingdrift. FIG. 1 shows the inlet 33 of the by-pass valve 28 connected tothe discharge line 36 of the compressor 25 and the outlet 39 of theby-pass valve 28 feeding into the suction line 31. This is the mostcommon method, however, many manufacturers choose to feed the outlet 39of the hot gas by-pass valve 28 into the inlet 41 of the evaporator 22.

Another commonly used method to maintain a constant refrigerant suctionpressure is to replace the expansion/restrictive device 27 and hot gasby-pass valve 28 with an automatic pressure valve (“APV”) 29, as shownin FIG. 2. The APV 29 maintains a constant refrigerant suction pressureby metering high-pressure liquid refrigerant into the inlet 41 of theevaporator 22. The APV 29 is typically inexpensive, but not highlyaccurate. Under no-load conditions, the liquid refrigerant may not beeffectively converted into a gas in the evaporator 22, which can resultin a liquid flood-back condition at the compressor suction, withpotential subsequent compressor 25 damage. Additionally, as the load isapplied to the dryer 15, the refrigerant suction pressure oftenincreases resulting in poor thermal performance.

A more recent technology used to maintain a constant refrigerant suctionpressure is the use of variable speed refrigerant compressors whichoperate by altering the rotational speed, and therefore, the pumpingcapacity of the compressor. To maintain a constant refrigerant suctionpressure, the rotational speed of the compressor can be increased ordecreased, to decrease or increase, respectively, the suction pressure.However, this design can require the use of power frequency inverters,suction line pressure sensors and/or temperature sensors.

Referring now to FIGS. 3 through 13 generally, a variable evaporatorcontrol (“VEC”) system and method for a refrigerated compressed gasdryer can be provided, according to the invention, based on anunderstanding of evaporator performance as discussed previously: thatis, under a full load condition, there is a need for either a largetemperature difference between the refrigerant and the outlet gas (andless required heat exchanger surface), or a large amount of low costsurface (and a precisely controlled refrigerant temperature), in orderto have a cost effective, thermally performing design. In addition,during periods of light load, or no load, the refrigerant temperature ina smooth tube-type evaporator must be accurately and constantlymaintained above the freezing point of water. In order to satisfy bothof these design constraints, a refrigerant control system can beprovided having sufficient intelligence to control, or vary, therefrigerant temperature as compressed gas loads are applied to andremoved from the dryer.

FIGS. 3.1 through 3.3 graphically illustrate the design criteriadescribed above and highlight the advantages of a VEC system forcontrolling, e.g., varying, the evaporator refrigerant temperature. FIG.3.1 shows the temperature profiles in the evaporator 22 of a typicalrefrigerated compressed gas dryer 15 as a function of the“characteristic length” of the evaporator 22. The graph assumes anevaporator 22 refrigerant temperature of 35 degrees Fahrenheit that isheld constant under all load conditions. A slight amount of refrigerantsuperheat is present at the refrigerant outlet 43 of the evaporator 22.This superheat is shown as 5 degrees Fahrenheit, and equates to arefrigerant exit temperature of 40 degrees Fahrenheit. Under a fulldesign flow condition, the warm compressed gas stream 17 is cooled froma temperature of 70 degrees Fahrenheit at the compressed gas inlet 46 ofthe evaporator 22 down to a temperature of 39 degrees Fahrenheit at thecompressed gas outlet 48 of the evaporator 22. Heat exchange occursalong the entire length of the evaporator 22. The length of theevaporator 22 in this case is defined, for comparative purposes, as 1.0(no unit designation). The approach temperature is 4 degrees Fahrenheit.The approach temperature is the difference between the refrigerant (atthe refrigerant inlet 41) and the compressed gas (at the compressed gasoutlet 48). The dotted line depicts the temperature profile along thelength of the evaporator 22 during periods of light load. Note that theapproach temperature remains nearly the same as the full load case, andthat the entire length of the evaporator 22 is not needed when reducedflows are present. This can be typical of current designs.

FIG. 3.2 shows similar information in type, but with a design that usesa lower evaporator 22 refrigerant temperature of 25 degrees Fahrenheit,and a required evaporator 22 characteristic length of approximately 0.5.The temperature of the cooled compressed gas stream 119, at thecompressed gas outlet 48, remains at 39 degrees Fahrenheit and theapproach temperature at the outlet 48 is 14 degrees Fahrenheit. This isan acceptable design for a full load condition. However, with currentcontrol technology and methods, the evaporator 22 refrigeranttemperature would remain constant at 25 degrees Fahrenheit as the loadis reduced., or removed. Consequently, if the compressed gas load dropsto the light load condition, the compressed gas stream will be cooled toa temperature below the freezing point of water, as the approachtemperature nears 4 degrees Fahrenheit. This can lead to a detrimentalcondensate freeze-up condition.

FIG. 3.3 illustrates operating characteristics which can be obtainedaccording to invention. Since the evaporator 22 refrigerant temperatureis readjusted as the load fluctuates from full design load to light andno load, the approach temperature is also readjusted, from 14 degreesFahrenheit under the full load condition, to 4 degrees Fahrenheit duringthe light load case. Therefore, the characteristic length of theevaporator 22 can be optimally sized for the full load condition (a 14degree Fahrenheit approach), resulting in a characteristic length whichis approximately 50% that of conventional designs. According to theinvention, the VEC system can cause the refrigerant temperature to riseas the load is reduced, thus maintaining a constant compressed gas exittemperature. Ultimately, the evaporator 22 refrigerant temperature isbrought above the freezing point of water, thereby safely eliminatingthe concern of condensate freeze-up.

One aspect of as presently preferred control method/system iscontrolling the cooling process using feedback from one or moretemperature and/or pressure sensors which can indicate the load on thecompressor. However, such sensors are not being used simply to maintaina constant refrigerant temperature as in the prior art. Rather, thesensors can be used to implement increased control over the system bymonitoring and adjusting the refrigerant temperature, and thus theapproach temperature, in the evaporator in order to generally maintainthe cooled gas exit temperature at a desired value. The prevailing loadon the compressor at a given time can be indirectly indicated by thefeedback from the sensors, and the temperature of the refrigerant can beadjusted accordingly, thereby adjusting the approach temperature, toavoid a potential freeze up condition at light or zero loads. Thus, byanalyzing the temperature sensor feedback, the microprocessor candetermine the compressor load, although, as explained above, themicroprocessor does not directly determine the magnitude of thecompressor load. Rather, by using the temperature sensor(s) properly andunderstanding the characteristics of the cooling system, there is noneed to know the load on the compressor. The compressed gas exittemperature will be generally maintained regardless of the compressorload.

Consequently, it can be understood that the cooling capabilityper-unit-length of a heat exchanger of any given fixed length can bemaximized by removing the conventional restriction of maintaining aconstant approach temperature irrespective of the load on thecompressor. The approach temperature can be large, i.e., the refrigeranttemperature low, when the load on the compressor is above a certainlevel, thus maximizing the amount of cooling possible for a given lengthheat exchanger. However, when the load on the compressor reduces below acertain level, a smaller approach temperature can be implemented, byincreasing the refrigerant temperature above freezing, to eliminate anypotential for freeze up.

A benefit of a VEC system according to the invention is that a low cost,smooth tube evaporator can successfully be utilized with largetemperature differences between the refrigerant and the compressed gasin order to satisfy the desired thermal and pressure drop performancecriteria while eliminating the potential of condensate freeze-up duringthe light load and no load conditions.

Hereinafter, will be described in detail certain presently preferredembodiments of VEC systems for refrigerated compressed gas dryingapplications. The following description of certain embodiments, asillustrated in FIGS. 4 through 13, are not intended to be exhaustive,but only representative of embodiments of VEC systems according to theinvention which can employ currently available supporting technology.

VEC Systems Utilizing an Electronic By-Pass Valve

As previously discussed, the evaporator 22 refrigerant temperature canbe controlled indirectly though the control of the refrigerant suctionpressure. Traditional technologies have conventionally utilizedmechanical pressure regulating valves, such as the hot gas by-pass valve28, which can be manually set to maintain a constant suction pressure.However, a new technology has emerged which places a small adjustmentmotor, e.g., a stepper motor, on to the hot gas by-pass valve 28 body,such as the motor 51 shown in FIG. 5, to provide for electronic controlof the by-pass valve 28. Electronic hot gas by-pass valves are availablemanufacturers such as Sporlan Valve Company, headquartered inWashington, Mo. Consequently, by electronic means, the setting of thismotor, and thus the by-pass valve 28 can be adjusted as required. Usingproper sensing techniques and microprocessor intelligence, theevaporator refrigerant temperature can be adjusted by adjusting thesuction pressure. In particular, using the motor 51 operated by-passvalve 28, the suction pressure can be increased to raise the evaporatorrefrigerant temperature above the freezing point of water whencompressed gas loads are removed. The evaporator refrigerant temperaturecan be measured at the evaporator inlet. Conversely, the suctionpressure can be decreased to lower the evaporator refrigeranttemperature as the load on the compressor is increased, simply bycontrolling the stepper motor 51 on the by-pass valve 28. Such aelectronically controllable by-pass valve can also be implemented withmany of the known refrigerant expansion/metering valves which controlthe refrigerant flow into the evaporator, such as a capillary tube, athermal expansion valve, an electronic expansion valve, or an orifice.

Sensing Techniques

Some sensing techniques which can be employed when using anelectronically controllable hot gas by-pass valve 28 as part of a VECsystem are described below.

A. Suction Pressure/Compressed Gas Temperature

A presently preferred embodiment of a VEC system 100 utilizing a by-passvalve 28 which is controllable electronically via motor 51 is shown inFIG. 4. As shown, a pressure sensor 54 can be used to monitor therefrigerant suction pressure in the suction line 31, and a temperaturesensor 57 can be used to monitor the compressed gas temperature at thecompressed gas outlet 48 of the evaporator 22. Output from the pressuresensor 54 and temperature sensor 57 can be supplied to a processor 60,such as a microprocessor, which can evaluate the information anddetermine any required adjustments to be made to the by-pass valve 28via motor 51 as the load on the refrigerant compressor 25 eitherincreases or decreases. The load on the refrigerant compressor 25 canvary due to changes in either the volume or the temperature of the warmcompressed gas 17 circulated through the evaporator 22. Specifically,the temperature of the compressed gas stream at the outlet 48 of theevaporator 22 can be monitored to generally maintain this temperature ata desired level. Since the compressed gas exit temperature can change ifthe load on the gas compressor changes, because the suction pressure ismaintained constant by the by-pass valve 28, the compressed gas exittemperature can be utilized to adjust the suction pressure using themotor 51 in order to maintain the compressed gas exit temperature at thedesired value. This can maximize the efficiency of the system andeliminate potential freeze up problems.

B. Refrigerant Temperature/Compressed Gas Temperature

Referring to FIG. 5, another embodiment of a VEC system 105 utilizing ahot gas by-pass valve 25 controllable electronically via motor 51 isshown. In this embodiment, instead of a pressure sensor on the suctionline 31, a first temperature sensor 63 can be used to monitor therefrigerant temperature at the evaporator 22 inlet 41. A secondtemperature sensor 66 can be used to monitor the compressed gastemperature at the compressed gas stream outlet 48 of the evaporator 22.As explained above, this information can be supplied to themicroprocessor 60 which can evaluate the information to determine therequired adjustment to be made to the by-pass valve 28 via the motor 51as the load on the compressor 25 increases or decreases.

C. Single Point Temperature

A further embodiment of a VEC system 110 is shown in FIG. 6, wherein thesystem can utilize a single point temperature sensing method. Thismethod can require determining an optimum sensing location for a singletemperature sensor 69 which can provide the temperature of thecompressed gas during periods of actual gas flow and also provide anaccurate evaporator 22 refrigerant temperature during periods of noflow. The microprocessor 60 can be supplied with this information andutilize it to determine the required adjustment to be made to theelectronically controllable by-pass valve 28 via motor 51 as the load onthe compressor 25 is increased or decreased.

FIG. 7 illustrates a particular embodiment of the single pointtemperature sensing method shown in FIG. 6, depicting an optimumlocation for, and presently preferred embodiment of, a single pointtemperature sensor 69. The evaporator 22 can be of a design utilizing amultiple smooth tube bundle 72 a-72 e enclosed in a single cover shell75. Compressed gas 18 flows through the tubes 72 a–72 e and therefrigerant 77 resides inside the cover shell 75. The end of the tubebundle 72 a–72 e can be isolated from the cover shell 75 with, forexample, a simple brazed tube sheet 78. The flow pattern is showncounter-flow, with the refrigerant 77 entering the evaporator 22 abovethe tube sheet 78 located near the compressed gas outlet tubes 72 a–72e. The refrigerant 77 exits the evaporator 22 at the opposite end of theevaporator 22, near the compressed gas inlet. As mentioned above, it canbe necessary to determine a physical location for the temperature sensor69 whereby the sensed temperature would be indicative of the compressedgas stream 18 temperature during periods of full and light flow, yetalso indicative of the refrigerant 77 temperature in the evaporator 22during a no load situation. Simply placing the temperature sensor 69directly in the gas stream 18 can satisfy the initial constraint quitewell, but when gas flow ceases, the temperature could rise in thestagnant gas environment, forcing the microprocessor 60 to lower therefrigerant 77 temperature. This result is opposite of the desiredeffect and can lead to a freeze-up condition. Conversely, by placing thetemperature sensor 69 directly in the refrigerant 77, or on the tubesheet 78, the temperature sensor 69 may respond appropriately during theno load condition, but may not behave correctly as a load is applied. Infact, the refrigerant 77 temperature may simply remain constant underall conditions.

Moreover, as further shown in FIG. 7, a presently preferred solution forimplementing a VEC system using single point temperature sensing caninclude placing a thermally conductive extension 80 on the end of one ofthe smooth tubes, e.g., tube 72 d, and then determining the appropriatetemperature sensor 69 position (labeled as “x”) which can accuratelyindicate the compressed gas stream 18 temperature when a light to fullflow is present (a combination of conductive and convective heattransfer). However, if placed too close to the tube sheet 78, thetemperature reading could be biased by conductive heat transfer into therefrigerant 77. Thus, the solution can further include inserting atemperature probe 83 through the wall of the cover shell 75 and assuringproper thermal contact with the outside surface of the extended tube,for example tube 72 d. In this manner, during a very light or zero flowcondition, a purely conductive heat transfer path can be establishedwith the evaporating refrigerant 77 above the tube sheet 78.

As a result, this solution can provide accurate temperature informationpermitting control over the system under all conditions by facilitatingan indication of the load, i.e., volume of warm compressed gas 17 beingcirculated through the evaporator 22. By knowing the volume ofcompressed gas being circulated, i.e., full or light load conditions,the approach temperature can be adjusted accordingly to enable maximumcooling for an evaporator 22 of any given length. For example, asillustrated in the graphs in FIGS. 3.1 through 3.3, a larger approachtemperature, i.e., a lower refrigerant inlet temperature, can beimplemented during a full load condition with no potential for freezeup. Conversely, a smaller approach temperature, i.e., a higherrefrigerant inlet temperature, can be provided during a light loadcondition to avoid potential freeze up.

A housing 84 can be provided through the cover shell 75 to the extension80, in which the temperature sensor 83 can be housed. Testing hasindicated that, using approximately 0.25 inch (outer diameter) smoothtubes 72 a–72 d, the proper distance, “x,” from the tube sheet 78 can beabout 0.25 inch. This distance has been satisfactory for various numbersof the smooth tubes 72 a–72 d, and different diameter cover shells 75.

In sum, the temperature of the compressed gas at the outlet 48 of theevaporator 22 dominates the sensor 83 reading when there is a light toheavy load on the compressor, and the refrigerant temperature dominateswhen there is a very light to zero load. Thus, a single-pointtemperature sensor, when placed in a proper location, can providesufficient feedback to the microprocessor to control the cooling systemregardless of the flow condition, i.e., the volume of warm compressedgas being circulated through the evaporator 22. For example, thecompressed gas exit temperature can be set at 37 or 38 degreesFahrenheit. If the compressed gas exit temperature increases, therefrigerant temperature will be permitted to drop until the 37 degreeFahrenheit temperature is satisfied. This is accomplished with no dangerof freeze up because the temperature of the compressed gas is stillbeing maintained above freezing even though the refrigerant temperaturemay fall below freezing at that set point. Then, if the compressor loaddrops off, the temperature detected by the sensor 83 will be dominatedby the temperature of the refrigerant, due to the conductive heattransfer path directly from the refrigerant. If the compressor load issignificantly reduced, the temperature of the compressed gas no longerdominates the temperature sensor 83; the refrigerant temperature nowdominates it. However, since the set point is maintained at about 37degrees Fahrenheit, the refrigerant temperature is permitted to riseabove the freezing point of water. When using multiple sensors, theprocessor may also be programmed with the appropriate logic andcomparative information between the two temperatures, i.e., compressedgas temperature versus refrigerant temperature, to properly control therefrigerant temperature.

VEC Systems Utilizing an Unloading-type Compressor

Another presently preferred embodiment of a VEC system can rely onvarying the capacity of the compressor to control the refrigerantsuction pressure, and corresponding evaporator refrigerant temperature.This can be realized through the use of unloading-type refrigerantcompressors. Whenever a lower suction pressure (lower evaporatorrefrigerant temperature) is desired, the capacity of the compressor canbe increased; conversely, as the need for increasing suction pressure(higher evaporator refrigerant temperature) is detected, the compressorcapacity can be decreased. This capacity control can be achieveddiscretely (i.e., fill capacity or no capacity) in some compressordesigns, such as the digital, or unloading, scroll compressor. Othermodels of multi-cylinder reciprocating compressors are designed topermit levels of capacity reduction, or capacity addition, in steps.Using various sensing techniques and the proper microprocessorintelligence, the suction pressure can therefore be raised as compressedgas loads are removed, or lowered as the load increases, by activatingthese unloading and loading mechanisms.

Sensing Techniques

Some sensing techniques which can be employed when using unloading typecompressors as part of a VEC system are described below.

A. Suction Pressure/Compressed Gas Temperature

A presently preferred embodiment of a VEC system 115 utilizing anunloading type compressor 86 is shown in FIG. 8. In this embodiment,similarly to the embodiment shown in FIG. 4, pressure sensor 54 can beused to monitor the refrigerant suction pressure at the suction line 31and temperature sensor 57 can be used to monitor the compressed gastemperature at the compressed gas outlet 48 of the evaporator 22. Themicroprocessor 60 can receive and evaluate this information to determinewhen to load or unload the compressor 86 in order to adjust the suctionpressure, and thus the evaporator 22 refrigerant temperature, as thecompressor 86 load is increased and decreased.

B. Refrigerant Temperature/Compressed Gas Temperature

Referring to FIG. 9, another embodiment of a VEC system 120 utilizing anunloading type compressor 86 is shown wherein, similarly to FIG. 5,first temperature sensor 63 can be used to monitor the refrigeranttemperature at the inlet 41 of the evaporator 22 and second temperaturesensor 66 can be used to monitor the compressed gas temperature at theevaporator 22 compressed gas stream outlet 48. The microprocessor 60then receives and utilizes this information to determine when to load orunload the compressor 86 in order to adjust the suction pressure as thecompressor 86 load is increased or decreased.

C. Single Point Temperature

Similarly to FIG. 6, FIG. 10 illustrates an embodiment of a VEC system125 utilizing an unloading type compressor 86 in a single pointtemperature sensing method. As explained previously, this method canrequire determining an optimum sensing location for the singletemperature sensor 69 which can accurately indicate both the temperatureof the compressed gas during periods of actual gas flow, and theevaporator 22 refrigerant temperature during periods of no flow. Themicroprocessor 60 can be supplied with this information which isevaluated to determine when to load or unload the compressor 86 in orderto adjust the suction pressure as the load on the compressor 86 isincreased or decreased. For details regarding locating an appropriatetemperature sensing location for the temperature sensor 69, refer to thedescription provided in connection with FIG. 7.

VEC Systems Utilizing a Variable Speed Compressor

As noted earlier, variable speed refrigerant compressors are availablewhich can vary the refrigeration capacity by altering the rotationalspeed of the compressor. This type of compressor can also be utilized inembodiments of a VEC system as a means to change the refrigerant suctionpressure as compressed gas loads are applied to and removed from thedryer. To increase the refrigerant suction pressure, the speed of thecompressor can be decreased; to decrease the refrigerant suctionpressure, the speed can be increased. Using various sensing techniquesand the proper microprocessor intelligence, the suction pressure can beraised as compressed gas loads are removed, and lowered as compressedgas loads increase, by controlling the rotational speed of thecompressor.

Sensing Techniques

Some sensing techniques which can be employed when using variable speedcompressors as part of a VEC system are described below.

A. Suction Pressure/Compressed Gas Temperature

FIG. 11 illustrates an embodiment of a VEC system 130 utilizing avariable speed compressor 90. In this embodiment, similarly to theembodiments of the invention shown in FIGS. 4 and 8, pressure sensor 54can monitor the refrigerant suction pressure at the suction line 31 andtemperature sensor 57 can monitor the compressed gas temperature at theoutlet 48 evaporator 22. This information can be supplied to themicroprocessor 60 which can evaluate the information and determinewhether to increase or decrease the rotational speed of the compressor90 to adjust the suction pressure, and thus the evaporator refrigeranttemperature, as the compressed gas load on the compressor 90 isincreased or decreased.

B. Refrigerant Temperature/Compressed Gas Temperature

Similarly to FIGS. 5 and 9, another embodiment of a VEC system 135 usinga variable speed compressor 90 is shown in FIG. 12. In this VEC system135, first temperature sensor 63 can monitor the evaporator 22refrigerant temperature at the inlet 41 to the evaporator 22 and secondtemperature sensor 66 can monitor the compressed gas temperature atoutlet 48 of the evaporator 22. This information can be supplied to themicroprocessor 60 for use in determining whether to increase or decreasethe rotational speed of the refrigerant compressor 90 in order to adjustthe suction pressure as the compressed gas load on the compressor 90 isincreased or decreased.

C. Single Point Temperature

Similarly to FIGS. 6 and 10, FIG. 13 illustrates a further embodiment ofa VEC system 140 using a single point temperature sensing method. Asexplained previously, this method can require determining an optimumlocation for the single temperature sensor 69 which will be indicativeof the compressed gas temperature during periods of actual gas flow, andwill also provide an accurate refrigerant evaporator 22 refrigeranttemperature during periods of no flow. The microprocessor 60 can beprovided with this information for use in and determining whether toincrease or decrease the rotational speed of the refrigerant compressor90 in order to adjust the suction pressure as the load on the compressor90 is increased or decreased. For details regarding locating anappropriate temperature sensing location, refer to the descriptionprovided in connection with FIG. 7.

As can be understood from the preceding description of certainembodiments of the invention, such a control system and method canpermit the use of smaller evaporators in conjunction with compressed gasdryers, which provides more efficient packaging, lower manufacturingcosts, and reduced pressure drop. Using the control system with smoothtube evaporator designs also permits non-fouling heat exchangeperformance, lower manufacturing costs and reduced pressure drop. Thecontrol system thus permits the use of compact heat exchanger designsemploying refrigerant temperatures below the freezing point of water(plate heat exchangers, bar and frame heat exchangers, etc.) without thedanger of condensate freeze-up at light load and no load conditions byadjusting the approach temperature according to changes in the warmcompressed gas load. Since the control system can respond to the actualcompressed gas temperature, proper and constant dryer performance andmoisture removal at all flow rates and conditions can be assured. Thecontrol system can also be embodied in many of the current technologiesavailable for refrigerant evaporator pressure/temperature control. Thesetechnologies may exist as control components, e.g., control valves, oras integral systems contained in the refrigerant compressors, such asunloading mechanisms, variable speed models, and the like.

Moreover, those of skill in the art will recognize that such a controlsystem according to the invention can also be adapted for applicationsin other areas of refrigeration and cooling. Accordingly, althoughcertain embodiments of the invention have been described in detail, itwill be appreciated by those skilled in the art that variousmodification to those details could be developed in light of the overallteaching of the disclosure. Therefore, the particular embodimentsdisclosed herein are intended to be illustrative only and not limitingto the scope of the invention which should be awarded the full breadthof the following claims and any and all embodiments thereof.

1. A variable evaporator control refrigerated compressed gas dryercomprising: a. an evaporator heat exchanger having a first flow path forcompressed gas at a first temperature; b. said evaporator heat exchangerhaving a second flow path for a refrigerant at a second temperaturewhich is lower than said first temperature to cool said compressed gasto a third temperature; c. a compressor to circulate said refrigerant ata predetermined pressure, said predetermined pressure having acorrelation to said second temperature; d. a temperature sensor to sensesaid third temperature at an outlet of said first flow path, whereinsaid temperature sensor positioned at a single location to sense afourth temperature wherein said fourth temperature is representative ofsaid third temperature when a compressed gas load is above a certainlevel, and representative of said second temperature when saidcompressed gas load is below said certain level; e. a valve intermediatesaid second flow path and a source of refrigerant, said valvecontrollable to admit refrigerant into circulation to adjust saidpredetermined pressure; and f. a controller connected to receivefeedback from said temperature sensor indicative of said thirdtemperature, said controller controlling said valve to adjust saidpredetermined pressure and thus adjust said second temperature togenerally maintain said third temperature at a desired value, whereinsaid controller connected to receive feedback from said temperaturesensor indicative of said fourth temperature to control said valve toadjust said predetermined pressure and thus said second temperature suchthat said fourth temperature is generally maintained at said desiredvalue; and g. a smooth tube evaporator heat exchanger.
 2. The compressedgas dryer of claim 1 wherein said smooth tube evaporator heat exchangerfurther comprises: a. an outer cover; b. a plurality of smooth tubesenclosed within said outer cover, said first flow path being at leastpartially through said plurality of smooth tubes; c. said second flowpath being within said outer cover and communicating with an exterior ofsaid plurality of smooth tubes; d. a tube sheet dividing a portion ofsaid evaporator heat exchanger, said second flow path communicating on afirst side of said tube sheet; e. each of said plurality of smooth tubeshaving an end extending through said tube sheet to a second side thereofwhich is not in communication with said second flow path; and f. saidsingle point temperature sensor being disposed in contact with said endof at least one of said plurality of smooth tubes which extends throughsaid tube sheet.
 3. The compressed gas dryer of claim 2 furthercomprising said end in contact with said single point temperature sensorhaving an extended length portion, and said single point temperaturesensor disposed in contact with said extended length portions.
 4. Avariable evaporator control system comprising: a. an evaporator heatexchanger having a first flow path for a gas at a first temperature anda second flow path for circulation of a refrigerant at a secondtemperature which is lower than said first temperature to cool said gasto a third temperature; b. a compressor to circulate said refrigerant;c. a temperature sensor positioned in said evaporator heat exchanger ata single location to sense a fourth temperature which is representativeof said third temperature when a gas load is above a certain level, andrepresentative of said second temperature when said gas load is belowsaid certain level; and d. a controller connected to receive feedbackfrom said temperature sensor indicative of said fourth temperature tocontrol said second temperature to generally maintain said fourthtemperature at a desired value.
 5. The variable evaporator controlsystem of claim 4 further comprising: a. compressor to circulate saidrefrigerant at a predetermined pressure, said second temperature havinga correlation to said predetermined pressure; and b. a valve positionedintermediate said second flow path and a source of refrigerant, saidvalve controllable by said controller to adjust admission of saidrefrigerant into circulation to adjust said predetermined pressure andthus adjust said second temperature to generally maintain said fourthtemperature at said desired value.
 6. A compressed gas dryer having aheat exchanger with a first flow path for a compressed gas and a secondflow path for a refrigerant to cool said compressed gas, the compressedgas dryer comprising: a. a temperature sensor to sense a temperature ofsaid compressed gas at an outlet of said first flow path; b. acontroller connected to receive feed back from said temperature sensorindicative of said temperature of said compressed gas at said outlet ofsaid first flow path, said controller controlling a temperature of saidrefrigerant at an inlet of said second flow path to generally maintainsaid temperature of said compressed gas at said outlet of said firstflow path at a desired value; c. said temperature sensor of saidcompressed gas at said outlet of said first flow path being positionedin said heat exchanger at a single location; d. a temperature sensed atsaid single location being representative of said temperature of saidcompressed gas at said outlet of said first flow path when a compressedgas load is above a certain level; and e. said temperature sensed atsaid single location being representative of said temperature of saidrefrigerant at said second flow when said compressed gas load is belowsaid certain level.
 7. The compressed gas dryer of claim 6 furthercomprising: said controller controlling said temperature of saidrefrigerant at said inlet of said second flow path to generallymaintaining said temperature sensed at said single location at saiddesired value.
 8. The compressed gas dryer of claim 7 wherein saiddesired value further comprises a value above 32 degrees Fahrenheit.